Friction drives are used in a variety of applications for converting a rotational input motion into a rotational output motion at a fixed or variable ratio. Power is transmitted through a friction drive via friction forces generated between two or more rotating components, and hence friction drives are effective only when there is essentially no slippage between the components. A limitation of all friction drives is that the friction force can be overcome at high loads, resulting in slippage. Since the coefficient of sliding friction is always less than the coefficient of static friction, once slippage starts to occur, a no-slip condition can be re-established only by either reducing the power input to the device until the transmitted force is less than the frictional force, or increasing the normal force between the components to increase the friction force above the transmitted force. However, reducing the power input is not desirable or feasible in many cases. Furthermore, increasing the normal force requires either constantly increasing the normal force across all use regimes, for instance by applying a preload force which acts between the components at all times, or sensing or anticipating when a slip occurs and temporarily increasing the normal force imparted on the rotating components. Both of these approaches have disadvantages. Constantly increasing the normal force results in increased rolling resistance, and hence decreased efficiency of the drive, even in low-load conditions where the increased normal force is not needed. Temporarily increasing the normal force requires additional equipment for sensing when a slip occurs or for making measurements of operating conditions and determining when a slip is likely to occur, as well as equipment for varying the normal force, resulting in greater complexity and increased cost of the friction drive.
Friction drives which naturally and automatically increase the friction force when the transmitted power increases have been developed as a means of delaying the onset of slippage to higher-load conditions. An example of such a drive is the V-belt transmission which is in use in the automotive field. The V-belt transmission employs flexible belts having a V-shaped cross-section and pulleys that have similarly V-shaped tracks within which the belts travel. Increasing load causes the belt to become more firmly wedged in the tracks so as to increase the frictional forces on the belt. Continuously variable transmissions (CVTs) employing the V-belt technology have been developed, including the Van Doorne transmission which employs pulleys having sheaves that are movable toward and away from each other for continuously varying the effective diameters of the pulleys so as to continuously vary an input-to-output speed ratio of the transmission.
However, a significant drawback of the V-belt design is that the belts tend to stretch and deform and undergo other physical changes which compromise the load-handling ability of the transmission. Transmission designers have attempted to overcome these problems of the V-belt transmission by developing a metal V-belt or chain which offers increased strength over rubber V-belts, and which transmits torque mainly via compressive forces acting between the metal links of the chain. However, an unfortunate characteristic of the metal V-belt is that slippage occurs between the metal links and the pulleys, and this slippage tends to increase sharply once the transmitted torque exceeds a certain value which is a function of the pulley clamping force on the belt. Thus, the problem of V-belt deformation has not been solved without also sacrificing one of the most important benefits of the V-belt design, namely, its natural friction-increasing characteristic.
No known friction-based CVT has been developed which has a natural and automatic tendency to increase the friction force when the input power and/or the load imposed on the output increases, and which does not require the use of relatively non-durable belts or the like.